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This month I have a bit of climate chat for you. As you can see from the accompanying photo of my ice-encrusted bivvy bag, my January sleep was chilly but not particularly wintery. Not only is there no snow at my bivvy spot, there is barely a scrap of the white stuff to be seen in the distant high Cairngorms.

Green Winters Return

Given Scotland’s geographical location, surrounded by sea heated by the Gulf Stream and often lashed by southwesterly winds, it is amazing how consistently good the winters of the 21st century have been so far. But always they are in the lap of chance, for every heavy snowfall that is consolidated into a durable base by a well timed freeze-thaw cycle, there are several that are washed away by rain and melted by warm winds. I have not yet experienced a winter in which this fate befell all the winter snows, but this could easily be my first. Indeed I seriously entertaining the possibility that we could be in for a green winter. Such disappointing seasons are nothing new – WH Murray wrote of the run of green winters of the 1920s, which together with the loss of so many men in the Great War, acted to stifle the development of Scottish winter climbing until the 1930s.

Longer term such green winters will certainly become the norm, and in the transition period anything might happen from a full arctic freeze to the type of winter we are experiencing now. Big changes are coming, and given the increasing unlikelihood of any action to curb carbon emissions, we are preparing for the effects, while hoping that somehow they won’t be as severe as some fear. And yet every indication is that far from overstating the dangers, we are understating them massively.

Out of the Ice House into the Hot House

Over geological time the climate has fluctuated wildly, from ice house states to hot house states. The present ice age (for we are still in an ice age, with extensive ice sheets at both poles and mountain glaciers around the world) has lasted 2.6 million years. This means that we humans – and our predecessors back to homo erectus – have known only an ice house state. The climate has varied from glacial stages where the ice sheets extend far south to interglacials where they retreat to something similar to present levels.

When I talk about hot house states I am not talking about the mild warmth of an interglacial. I am talking about a world so warm that the it would be unrecognisable to us ice age humans.

The difference between a hot house and an ice house is well illustrated by the case of Ellesmere Island, the heavily indented island that sits to the west of northern Greenland, a few hundred miles form the North Pole. While the continents have moved round the globe over deep time, Ellesmere island has stayed put, caught in a tectonic eddie. Today the winter temperatures drop to -40 C and the warmest summer days creep up to only 20 C. The only vegetation is dwarf willow and the island is populated by musk ox, caribou seals and polar bears.

The island was explored by the ill-fated Greely expedition in 1882, which ended up in starvation and cannibalism. They discovered evidence of a very different and much warmer past in which alligators, giant tortoise and ancient hippo like animals walked in a vast forest of sixty metre tall redwoods. The summer temperatures were similar to today but the winter temperatures only dropped to around zero, balmy for the poles. This was no isolated blip, the world was this warm throughout time times when the dinosaurs roamed the earth.

The disturbing message of the fossil leaves

The temperature of the earth is largely determined by the carbon dioxide concentration. It acts as a thermostat, with higher temperatures observed during times of high CO2. This effect is clear during our ice house world. We have direct measurements from bubbles in ice cores from Antarctica and Greenland which go back to 800,000 years ago and show that period was ice house switching between 180 and 280 parts per million (ppm) CO2. Until recently, estimates of CO2 concentration in the hot house climate of the past ranged widely, with some sources suggesting they may have been up to 6000 ppm.

Now it would be reassuring if the CO2 levels during these warm periods were much, much higher than where we will end up in the near future, because this would suggest that we could continue to pump out CO2 while continuing to live on and feed ourselves from a planet not so different from that of today.

I read recently of new, more accurate, estimates of historical CO2 levels based on the analysis of fossil leaves from Ellesmere Island and elsewhere. Just as we breath oxygen, plants suck carbon dioxide through pores called stomata on the underside of their leaves. And just as high altitude peoples have more powerful lungs so they can extract enough oxygen, so plants in a low CO2 environment have larger stomata. This means that if you know the size of the stomata in the past you can estimate the CO2 levels in the atmosphere.

So what is the message from the fossil leaves? They show that 66 milion years ago, just after the demise of the dinosaurs, when the Ellesmere Island was home to crocodiles instead of polar bears, the CO2 level was 650 ppm. Going back into the geological past as far as 400 MA shows that CO2 levels have never exceeded 1000 ppm. This is a sobering finding, for CO2 levels just passed 400 ppm and will hit 600 ppm in the mid-late 21st century, even if we reduce our CO2 output. 1000 ppm is where we will get to by 2100 under ‘business as usual’ assumptions. With Trump in the White House even ‘business as usual’ looks optimistic – millions of years of dark-adapted redwoods growing in the poles has led to the development of substantial coal reserves in the Arctic, which I fear Trump will crack into with all the thuggish glee of an IS fighter dynamiting the ruins of Palmyra. Both the past and the future are under threat this post-truth age.

So there you have it. The end destination of the journey that we are all on is a world that is very different to the one in which our civilisation has developed, very different to the world that all humans have experienced. There will not only be green winters in Scotland, but green winters in the polar regions.

It’s impossible to say what this all means for ski conditions in Scotland over the coming decades, for we have little idea how long the climate system will take to respond to the massive injection of CO2 we are giving it. The winters could go green within decades, but equally we may experience runs of severe winters as climate variability increases*. Literally anything could happen in the short term, but one thing seems certain: future generations will have more pressing concerns than their declining opportunities to participate in winter sports.

* It is likely that the relatively severe winters that we have enjoyed of late are a counterintuitive side effect of the warming planet. The polar jet stream separates the cold air of the Arctic from the warmer air of the temperate latitudes. This jet stream is driven by the temperature difference between the poles and the temperate regions below. As the Arctic warms this temperature gradient decreases, robbing the jet stream of strength. Rather than appearing as a band that neatly circles the pole, it begins to meander, in extreme cases resembling a clover leaf when viewed from above, with warm air incursions into the Arctic and cold polar air sliding south to bring a bit of the Arctic to lower latitudes. This is great when we get the cold air as we did spectacularly in 2010/11, less so when we are deprived of it and instead get fed a constant diet of mild, wet Atlantic air, converting our winter into a darker version of a run of bad summer days.